Development of a net ozone production rate detection system based on dual-channel cavity ring-down spectroscopy

A novel system for measuring net photochemical ozone production rates in the atmosphere based on cavity ring-down spectroscopy (OPR-CRDS) was developed. The system consists of two chambers (a reaction chamber and a reference chamber) and a dual-channel Ox-CRDS detector. To minimize the wall loss of Ox in the chambers, the inner surfaces of both chambers are coated with Teflon film. The performance of the OPR-CRDS system was characterized. It was found that even though the photolysis frequency (J value) decreased by 10%, the decrease in the P(O3) caused by the ultraviolet-blocking film coating was less than 3%. The two chambers had a good consistency in the mean residence time and the measurement of NO2 and Ox under the condition of no sunlight. The detection limit of the OPR-CRDS was determined to be 0.20 ppbv/hr. To further verify the accuracy of the system, the direct measurement values of the OPR-CRDS system were compared with the calculation results based on radical (OH, HO2, and RO2) reactions, and a good correlation was obtained between the measured and calculated values. Finally, the developed instrument was applied to obtain the comprehensive field observations at an urban site in the Yangtze River Delta (China) for 40 days, the time series and change characteristics of the P(O3) were obtained directly, and the good environmental adaptability and stability of the OPR-CRDS system were demonstrated. It is expected that the new instrument will be beneficial to investigations of the relationship between P(O3) and its precursors.

Unclassical Radical Generation Mechanisms in the Troposphere: A Review

Hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively known as HOx radicals, are crucial in removing primary pollutants, controlling atmospheric oxidation capacity, and regulating global air quality and climate. An imbalance between radical observations and simulations has been identified based on radical closure experiments, a valuable tool for accessing the state-of-the-art chemical mechanisms, demonstrating a deviation between the existing and actual tropospheric mechanisms. In the past decades, researchers have attempted to explain this deviation and proposed numerous radical generation mechanisms. However, these newly proposed unclassical radical generation mechanisms have not been systematically reviewed, and previous radical-related reviews dominantly focus on radical measurement instruments and radical observations in extensive field campaigns. Herein, we overview the unclassical generation mechanisms of radicals, mainly focusing on outlining the methodology and results of radical closure experiments worldwide and systematically introducing the mainstream mechanisms of unclassical radical generation, involving the bimolecular reaction of HO2 and organic peroxy radicals (RO2), RO2 isomerization, halogen chemistry, the reaction of H2O with O2 over soot, epoxide formation mechanism, mechanism of electronically excited NO2 and water, and prompt HO2 formation in aromatic oxidation. Finally, we highlight the existing gaps in the current studies and suggest possible directions for future research. This review of unclassical radical generation mechanisms will help promote a comprehensive understanding of the latest radical mechanisms and the development of additional new mechanisms to further explain deviations between the existing and actual mechanisms.

Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China

Ambient peroxy radical (RO2⁎ = HO2 + RO2) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O3 pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO2⁎ concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO2⁎ by a mean factor of 7.2. This underprediction can be explained by a missing RO2⁎ source of 1.2 ppbv h-1 which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of ROx (= OH + HO2 + RO2) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O3 and HONO (25 %), and HCHO (24 %). And the major sinks of ROx transitioned from a predominant reaction of radicals with NOx in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO2 radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.

Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?

Despite several investigations on the atmospheric fate of cyclic volatile methyl siloxanes (VMS), the oxidation chemistry of these purely anthropogenic, high production volume compounds is poorly understood. This led to uncertainties in the environmental impact and fate of the oxidation products. According to laboratory measurements, the main VMS oxidation product is the siloxanol (a -CH3 replaced with an -OH); however, none of the mechanisms proposed to date satisfactorily explain its formation. Motivated by our previous experimental observations of VMS oxidation products, we use theoretical quantum chemical calculations to (1) explore a previously unconsidered reaction pathway to form the siloxanol from a reaction of a siloxy radical with gas-phase water, (2) investigate differences in reaction rates of radical intermediates in hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) oxidation, and (3) attempt to explain the experimentally observed products. Our results suggest that while the proposed reaction of the siloxy radical with water to form the siloxanol can occur, it is too slow to compete with other unimolecular reactions and thus cannot explain the observed siloxanol formation. We also find that the reaction between the initial D3 peroxy radical (RO2•) with HO2• is slower than previously anticipated (calculated as 3 × 10-13 cm3 molecule-1 s-1 for D3 and 2 × 10-11 cm3 molecule-1 s-1 for D4 compared to the general rate of ∼1 × 10-11 cm3 molecule-1 s-1). Finally, we compare the anticipated fates of the RO2• under a variety of conditions and find that a reaction with NO (assuming a general RO2• + NO bimolecular rate constant of 9 × 10-12 cm3 molecule-1 s-1) will likely be the dominant fate in urban conditions, while isomerization can be important in cleaner environments.

Photochemical evolution of air in a tropical urban environment of India: A model-based study

The photochemical processes over tropical Indian region impact the atmospheric composition and air quality over local to global scales; nevertheless, studies on detailed atmospheric chemistry remain sparse in this region. In this study, we investigate the photochemical evolution of air in the downwind of a tropical semi-arid urban environment (Ahmedabad) in India using the Master Mechanism model. The 5-days long chemical evolution has been simulated for the winter conditions - when this region experiences strong ozone build up. Model environment has been set up by including the meteorological conditions, overhead ozone, and aerosol loading, etc. Nitrogen oxides (NO_x), carbon monoxide (CO), ozone (O₃), and several volatile organic compounds (VOCs) have been initialized in the model based on the wintertime observations. The model predicts large O₃ production (∼115 ppbv) in the downwind regions, followed by a gradual decrease from the 3^rd day onwards. Additionally, significant amounts of the secondary inorganics, e.g. nitric acid (∼17 ppbv), hydrogen peroxide (∼9 ppbv), and organics, e.g. ketones (∼11 ppbv), are also simulated. The noontime maximum levels of hydroxyl (OH) and hydroperoxyl (HO₂) radicals are simulated to be 0.3 and 44 pptv, respectively. While the production of OH is dominated by the reaction of NO with HO₂ on the first day, photolysis of O₃ dominates subsequently with reduction in NO_x levels. VOCs are the major OH sink during day 1, however contribution of CO is greater on further days. The air mass trajectory analysis suggests the outflow of ozone-rich air over the rural areas and the Arabian Sea, in agreement with measurements and a global model. Our study highlights the strong impact of the urban outflows on the regional atmospheric composition. The continuous measurements of VOCs and radicals are needed over tropical regions to complement the models and further improve the understanding of air chemistry.

Direct evidence of local photochemical production driven ozone episode in Beijing: A case study

We present a comprehensive field campaign conducted in Beijing, September 2016, to elucidate the photochemical smog pollution, i.e. Ozone (O₃). The observed daily maximum hydroxyl radical (OH) and hydroperoxy radical (HO₂) concentrations were up to 1 × 10⁷ cm^-3 and 6 × 10⁸ cm^-3, respectively, indicating the active photochemistry in autumn Beijing. Photolysis of nitrous acid (HONO) and O₃ contributed 1-2 ppbv h^-1 to OH primary production during daytime. OH termination were dominated by the reaction with nitric oxide (NO) and nitrogen dioxide (NO₂), which were in general larger than primary production rates, indicating other primary radical sources maybe important. The measurement of radicals facilitates the direct determination of local ozone production rate P (O_x) (O_x = O₃ + NO₂). The integrated P(O_x) reached 75 ppbv in afternoon (for 4 h) when planetary boundary layer was well developed. At the same time period, the observed total oxidant concentrations O_x, increased significantly by 70 ppbv. In addition, the O_x measurement showed compact increase in 12 stations both temporally and spatially in Beijing, indicating that active photochemical production happened homogenously throughout the city. The back-trajectory analysis showed that Beijing was isolated from the other cities during the episode, which further proved that the fast ozone pollution was contributed by local photochemical production rather than regional advection.

The use of stable oxygen and nitrogen isotopic signatures to reveal variations in the nitrate formation pathways and sources in different seasons and regions in China

Nitrate (NO₃^-) is one of the most important inorganic ions in fine particulate (PM_2.5) and drives regional haze formation; however, the NO₃^- sources and formation mechanisms in different seasons and regions are still debated. Here, PM_2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO₃^- (δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^-), estimated the δ¹⁸O-HNO₃ values produced by different oxidation pathways, and quantified the NO₃^- formation pathways based on the isotope mass-balance equation. Our results showed that the δ¹⁸O-NO₃^- values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ¹⁸O-HNO₃ values arising from the OH radical pathway (P_OH, 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ¹⁸O-NO₃^- values are mainly influenced by P_OH, which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO₃ produced by P_OH and reduced the amount of HNO₃ produced by P_N2O5, which produced low δ¹⁸O-NO₃^- values. Increased air pollution emissions decreased the contribution from P_OH and increased the contribution from N₂O₅ and NO₃ pathways (P_N2O5+NO3). The δ¹⁵N-NO₃^- values of PM_2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NO_x emissions with positive isotopic values led to high δ¹⁵N-NO₃^- values in northern China and during cold months. A higher f_NO2 (f_NO2 = NO₂/(NO + NO₂), temperature, and contribution of P_OH produced lower N isotope fractionation between NO_x and δ¹⁵N-NO₃^-, which was found to further decrease the δ¹⁵N-NO₃^- values in southwestern China and during warm months.

A modelling study of OH, NO3 and H2SO4 in 2007-2018 at SMEAR II, Finland: analysis of long-term trends

Major atmospheric oxidants (OH, O₃ and NO₃) dominate the atmospheric oxidation capacity, while H₂SO₄ is considered as a main driver for new particle formation. Although numerous studies have investigated the long-term trend of ozone in Europe, the trends of OH, NO₃ and H₂SO₄ at specific sites are to a large extent unknown. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station and has been validated by measurements in several projects. Here, we applied the SOSAA model for the years 2007-2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants OH and NO₃, as well as H₂SO₄ at SMEAR II. The simulations were evaluated with observations from several shorter and longer campaigns at SMEAR II. Our results show that daily OH increased by 2.39% per year and NO₃ decreased by 3.41% per year, with different trends of these oxidants during day and night. On the contrary, daytime sulfuric acid concentrations decreased by 2.78% per year, which correlated with the observed decreasing concentration of newly formed particles in the size range of 3-25 nm with 1.4% per year at SMEAR II during the years 1997-2012. Additionally, we compared our simulated OH, NO₃ and H₂SO₄ concentrations with proxies, which are commonly applied in case a limited number of parameters are measured and no detailed model simulations are available.

Variability and Time of Day Dependence of Ozone Photochemistry in Western Wildfire Plumes

Understanding the efficiency and variability of photochemical ozone (O₃) production from western wildfire plumes is important to accurately estimate their influence on North American air quality. A set of photochemical measurements were made from the NOAA Twin Otter research aircraft as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment. We use a zero-dimensional (0-D) box model to investigate the chemistry driving O₃ production in modeled plumes. Modeled afternoon plumes reached a maximum O₃ mixing ratio of 140 ± 50 ppbv (average ± standard deviation) within 20 ± 10 min of emission compared to 76 ± 12 ppbv in 60 ± 30 min in evening plumes. Afternoon and evening maximum O₃ isopleths indicate that plumes were near their peak in NO_x efficiency. A radical budget describes the NO_x volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NO_x-sensitive chemistry, driven by HO_x (=OH + HO₂) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HO_x) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NO_x efficiency to VOC-sensitive O₃ production caused by a reduction in photolysis rates and fire emissions. HO_x production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).

Observations and modeling of OH and HO2 radicals in Chengdu, China in summer 2019

This study reports on the first continuous measurements of ambient OH and HO₂ radicals at a suburban site in Chengdu, Southwest China, which were collected during 2019 as part of a comprehensive field campaign 'CompreHensive field experiment to explOre the photochemical Ozone formation mechaniSm in summEr - 2019 (CHOOSE-2019)'. The mean concentrations (11:00-15:00) of the observed OH and HO₂ radicals were 9.5 × 10⁶ and 9.0 × 10⁸ cm^-3, respectively. To investigate the state-of-the-art chemical mechanism of radical, closure experiments were conducted with a box model, in which the RACM2 mechanism updated with the latest isoprene chemistry (RACM2-LIM1) was used. In the base run, OH radicals were underestimated by the model for the low-NO regime, which was likely due to the missing OH recycling. However, good agreement between the observed and modeled OH concentrations was achieved when an additional species X (equivalent to 0.25 ppb of NO mixing ratio) from one new OH regeneration cycle (RO₂ + X → HO₂, HO₂ + X → OH) was added into the model. Additionally, in the base run, the model could reproduce the observed HO₂ concentrations. Discrepancies in the observed and modeled HO₂ concentrations were found in the sensitivity runs with HO₂ heterogeneous uptake, indicating that the impact of the uptake may be less significant in Chengdu because of the relatively low aerosol concentrations. The ROx (= OH + HO₂ + RO₂) primary source was dominated by photolysis reactions, in which HONO, O₃, and HCHO photolysis accounted for 34%, 19%, and 23% during the daytime, respectively. The efficiency of radical cycling was quantified by the radical chain length, which was determined by the NO to NO₂ ratio successfully. The parameterization of the radical chain length may be very useful for the further determinations of radical recycling.

Identification of close relationship between atmospheric oxidation and ozone formation regimes in a photochemically active region

Understanding ozone (O3) formation regime is a prerequisite in formulating an effective O3 pollution control strategy. Photochemical indicator is a simple and direct method in identifying O3 formation regimes. Most used indicators are derived from observations, whereas the role of atmospheric oxidation is not in consideration, which is the core driver of O3 formation. Thus, it may impact accuracy in signaling O3 formation regimes. In this study, an advanced three-dimensional numerical modeling system was used to investigate the relationship between atmospheric oxidation and O3 formation regimes during a long-lasting O3 exceedance event in September 2017 over the Pearl River Delta (PRD) of China. We discovered a clear relationship between atmospheric oxidative capacity and O3 formation regime. Over eastern PRD, O3 formation was mainly in a NOx-limited regime when HO2/OH ratio was higher than 11, while in a VOC-limited regime when the ratio was lower than 9.5. Over central and western PRD, an HO2/OH ratio higher than 5 and lower than 2 was indicative of NOx-limited and VOC-limited regime, respectively. Physical contribution, including horizontal transport and vertical transport, may pose uncertainties on the indication of O3 formation regime by HO2/OH ratio. In comparison with other commonly used photochemical indicators, HO2/OH ratio had the best performance in differentiating O3 formation regimes. This study highlighted the necessities in using an atmospheric oxidative capacity-based indicator to infer O3 formation regime, and underscored the importance of characterizing behaviors of radicals to gain insight in atmospheric processes leading to O3 pollution over a photochemically active region.

Total hydroxyl radical reactivity measurements in a suburban area during AQUAS-Tsukuba campaign in summer 2017

Missing hydroxyl radical (OH) reactivity from unknown/unmeasured trace species empirically accounts for 10%-30% of total OH reactivity and may cause significant uncertainty regarding estimation of photochemical ozone production. Thus, it is essential to unveil the missing OH reactivity for developing an effective ozone mitigation strategy. In this study, we conducted simultaneous observations of total OH reactivity and 54 reactive trace species in a suburban area as part of the Air QUAlity Study (AQUAS)-Tsukuba campaign for the summer of 2017 to gain in-depth insight into total OH reactivity in an area that experienced relatively high contributions of secondary pollutants. The campaign identified on average 35.3% of missing OH reactivity among total OH reactivity (12.9 s^-1). In general, ozone-production potential estimation categorized ozone formation in this area as volatile organic compound (VOC)-limited conditions, and missing OH reactivity may increase ozone production potential 40% on average if considered. Our results suggest the importance of photochemical processes of both AVOCs and BVOCs for the production of missing OH reactivity and that we may underestimate the importance of reducing precursors in approach to suppressing ozone production if we ignore the contribution of their photochemical products.

Winter photochemistry in Beijing: Observation and model simulation of OH and HO2 radicals at an urban site

A field campaign was conducted from November to December 2017 at the campus of Peking University (PKU) to investigate the formation mechanism of the winter air pollution in Beijing with the measurement of hydroxyl and hydroperoxyl radical (OH and HO₂) with the support from comprehensive observation of trace gases compounds. The extent of air pollution depends on meteorological conditions. The daily maximum OH radical concentrations are on average 2.0 × 10⁶ cm^-3 and 1.5 × 10⁶ cm^-3 during the clean and polluted episodes, respectively. The daily maximum HO₂ radical concentrations are on average 0.4 × 10⁸ cm^-3 and 0.3 × 10⁸ cm^-3 during the clean and polluted episodes, respectively (diurnal averaged for one hour bin). A box model based on RACM2-LIM1 mechanism can reproduce the OH concentrations but underestimate the HO₂ concentrations by 50% during the clean episode. The OH and HO₂ concentrations are underestimated by 50% and 12 folds during the polluted episode, respectively. Strong dependence on nitric oxide (NO) concentration is found for both observed and modeled HO₂ concentrations, with the modeled HO₂ decreasing more rapidly than observed HO₂, leading to severe HO₂ underestimation at higher NO concentrations. The OH reactivity is calculated from measured and modeled species and inorganic compounds (carbon monoxide (CO), NO, and nitrogen dioxide (NO₂)) make up 69%-76% of the calculated OH reactivity. The photochemical oxidation rate denoted by the OH loss rate increases by 3 times from the clean to polluted episodes, indicating the strong oxidation capacity in polluted conditions. The comparison between measurements at PKU site and a suburban site from one previous study shows that chemical conditions are similar in both urban and suburban areas. Hence, the strong oxidation capacity and its potential contribution to the pollution bursts are relatively homogeneous over the whole Beijing city and its surrounding areas.

Exploring atmospheric free-radical chemistry in China: the self-cleansing capacity and the formation of secondary air pollution

Since 1971, it has been known that the atmospheric free radicals play a pivotal role in maintaining the oxidizing power of the troposphere. The existence of the oxidizing power is an important feature of the troposphere to remove primary air pollutants emitted from human beings as well as those from the biosphere. Nevertheless, serious secondary air-pollution incidents can take place due to fast oxidation of the primary pollutants. Elucidating the atmospheric free-radical chemistry is a demanding task in the field of atmospheric chemistry worldwide, which includes two kinds of work: first, the setup of reliable radical detection systems; second, integrated field studies that enable closure studies on the sources and sinks of targeted radicals such as OH and NO3. In this review, we try to review the Chinese efforts to explore the atmospheric free-radical chemistry in such chemical complex environments and the possible link of this fast gas-phase oxidation with the fast formation of secondary air pollution in the city-cluster areas in China.

Cholesterol provides nonsacrificial protection of membrane lipids from chemical damage at air-water interface

The role of cholesterol in bilayer and monolayer lipid membranes has been of great interest. On the biophysical front, cholesterol significantly increases the order of the lipid packing, lowers the membrane permeability, and maintains membrane fluidity by forming liquid-ordered-phase lipid rafts. However, direct observation of any influence on membrane chemistry related to these cholesterol-induced physical properties has been absent. Here we report that the addition of 30 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) monolayers at the air-water interface greatly reduces the oxidation and ester linkage cleavage chemistries initiated by potent chemicals such as OH radicals and HCl vapor, respectively. These results shed light on the indispensable chemoprotective function of cholesterol in lipid membranes. Another significant finding is that OH oxidation of unsaturated lipids generates Criegee intermediate, which is an important radical involved in many atmospheric processes.

Southeast Atmosphere Studies: learning from model-observation syntheses

Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations, but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods, during which collocated measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013 provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales. This paper is aimed at discussing progress in evaluating, diagnosing and improving air quality and climate modeling using comparisons to SAS observations as a guide to thinking about improvements to mechanisms and parameterizations in models. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosol (SOA) and other trace species in the troposphere, with the ultimate goal of understanding the radiative impacts of these species in the southeast and elsewhere. Here we address questions surrounding four key themes: gas-phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. We expect this review to serve as a guidance for future modeling efforts.

Identification of chemical fingerprints in long-range transport of burning induced upper tropospheric ozone from Colorado to the North Atlantic Ocean

This study investigates a significant biomass burning (BB) event occurred in Colorado of the United States in 2012 using the Community Multi-scale Air Quality (CMAQ) model. The simulation reasonably reproduced the significantly high upper tropospheric O₃ concentrations (up to 145ppb) caused by BB emissions. We find the BB-induced O₃ was primarily affected by chemical reactions and dispersion during its transport. In the early period of transport, high NO_x and VOCs emissions caused O₃ production due to reactions with the peroxide and hydroxyl radicals, HO₂ and OH. Here, NO_x played a key role in O₃ formation in the BB plume. The results indicated that HO₂ in the BB plume primarily came from formaldehyde (HCHO+hv=2HO₂+CO), a secondary alkoxy radical (ROR=HO₂). CO played an important role in the production of recycled HO₂ (OH+CO=HO₂) because of its abundance in the BB plume. The chemically produced HO₂ was largely converted to OH by the reactions with NO (HO₂+NO=OH+NO₂) from BB emissions. This is in contrast to the surface, where HO₂ and OH are strongly affected by VOC and HONO, respectively. In the late stages of transport, the O₃ concentration was primarily controlled by dispersion. It stayed longer in the upper troposphere compared to the surface due to sustained depletion of NO_x. Sensitivity analysis results support that O₃ in the BB plume is significantly more sensitive to NO_x than VOCs.

Oxidizing capacity of the rural atmosphere in Hong Kong, Southern China

Atmospheric oxidizing capacity (AOC), dominated by the hydroxyl radical (OH), is an important index of the self-cleaning capacity of atmosphere and plays a vital role in the tropospheric chemistry. To better understand the key processes governing the chemistry of rural atmosphere of southern China, we analyzed the oxidation capacity and radical chemistry at a regional background site in Hong Kong from 23 August to 22 December 2012, which covered the summer, autumn and winter seasons. A chemical box model built on the latest Master Chemical Mechanism (v3.3) was used to elucidate the OH reactivity and sources of RO_X radicals (RO_X=OH+HO₂+RO₂). The AOC showed a clear seasonal pattern with stronger intensity in late summer compared to autumn and winter. Reactions with NO₂ (30%) and oxygenated volatile organic compounds (OVOCs) (31%) together dominated the OH loss in summer, while reactions with CO (38% in autumn and 39% in winter) and OVOCs (34% in autumn and 25% in winter) made larger contributions in autumn and winter. Photolysis of O₃ (36%-47%) presented the major RO_X source during all three seasons. The second largest ROx source was HONO photolysis (25%) in summer compared to HCHO photolysis in autumn (20%) and winter (21%). Besides, photolysis of other OVOCs was another important primary source of ROx radicals with average contributions of 14%, 13% and 20% for the summer, autumn and winter cases, respectively. Overall, the present study evaluates the oxidizing capacity of the rural atmosphere of South China and elucidates the varying characteristics of photochemical processes in different air masses.

Derivation of Hydroperoxyl Radical Levels at an Urban Site via Measurement of Pernitric Acid by Iodide Chemical Ionization Mass Spectrometry

Hydroperoxyl radical (HO₂) is a key species to atmospheric chemistry. At warm temperatures, the HO₂ and NO₂ come to a rapid steady state with pernitric acid (HO₂NO₂). This paper presents the derivation of HO₂ from observations of HO₂NO₂ and NO₂ in metropolitan Atlanta, US, in winter 2014 and summer 2015. HO₂ was observed to have a diurnal cycle with morning concentrations suppressed by high NO from the traffic. At night, derived HO₂ levels were nonzero and exhibited correlations with O₃ and NO₃, consistent with previous studies that ozonolysis and oxidation by NO₃ are sources of nighttime HO₂. Measured and model calculated HO₂ were in reasonable agreement: Without the constraint of measured HO₂NO₂, the model reproduced HO₂ with a model-to-observed ratio (M/O) of 1.27 (r = 0.54) for winter, 2014, and 0.70 (r = 0.80) for summer, 2015. Adding measured HO₂NO₂ as a constraint, the model predicted HO₂ with M/O = 1.13 (r = 0.77) for winter 2014 and 0.90 (r = 0.97) for summer 2015. These results demonstrate the feasibility of deriving HO₂ from HO₂NO₂ measurements in warm regions where HO₂NO₂ has a short lifetime.

Investigating the evolution of summertime secondary atmospheric pollutants in urban Beijing

Understanding the formation of tropospheric ozone (O₃) and secondary particulates is essential for controlling secondary pollution in megacities. Intensive observations were conducted to investigate the evolution of O₃, nitrate (NO₃^-), sulfate (SO₄^2-) and oxygenated organic aerosols ((OOAs), a proxy for secondary organic aerosols) and the interactions between O₃, NO_x oxidation products (NO_z) and OOA in urban Beijing in August 2012. The O₃ concentrations exhibited similar variations at both the urban and urban background sites in Beijing. Regarding the O₃ profile, the O₃ concentrations increased with increasing altitude. The peaks in O₃ on the days exceeding the 1h or 8h O₃ standards (polluted days) were substantially wider than those on normal days. Significant increases in the NO_z mixing ratio (i.e., NO_y - NO_x) were observed between the morning and early afternoon, which were consistent with the increasing oxidant level. A discernable NO₃^- peak was also observed in the morning on the polluted days, and this peak was attributed to vertical mixing and strong photochemical production. In addition, a SO₄^2- peak at 18:00 was likely caused by a combination of local generation and regional transport. The OOA concentration cycle exhibited two peaks at approximately 10:00 and 19:00. The OOA concentrations were correlated well with SO₄^2- ([OOA]=0.55×[SO₄^2-]+2.1, r²=0.69) because they both originated from secondary transformations that were dependent on the ambient oxidization level and relative humidity. However, the slope between OOA and SO₄^2- was only 0.35, which was smaller than the slope observed for all of the OOA and SO₄^2- data, when the RH ranged from 40 to 50%. In addition, a photochemical episode was selected for analysis. The results showed that regional transport played an important role in the evolution of the investigated secondary pollutants. The measured OOA and O_x concentrations were well correlated at the daily scale, whereas the hourly OOA and O_x concentrations were insignificantly correlated in urban Beijing. The synoptic situation and the differences in the VOC oxidation contributing to O₃ and SOAs may have resulted in the differences among the correlations between OOA and O_x at different time scale. We calculated OOA production rates using the photochemical age (defined as -log₁₀(NO_x/NO_y)) in urban plumes. The CO-normalized OOA concentration increased with increasing photochemical age, with production rates ranging from 1.1 to 8.5μgm^-3ppm^-1h^-1 for the plume from the NCP.

Ozone production chemistry in the presence of urban plumes

Ozone pollution affects human health, especially in urban areas on hot sunny days. Its basic photochemistry has been known for decades and yet it is still not possible to correctly predict the high ozone levels that are the greatest threat. The CalNex_SJV study in Bakersfield CA in May/June 2010 provided an opportunity to examine ozone photochemistry in an urban area surrounded by agriculture. The measurement suite included hydroxyl (OH), hydroperoxyl (HO₂), and OH reactivity, which are compared with the output of a photochemical box model. While the agreement is generally within combined uncertainties, measured HO₂ far exceeds modeled HO₂ in NO_x-rich plumes. OH production and loss do not balance as they should in the morning, and the ozone production calculated with measured HO₂ is a decade greater than that calculated with modeled HO₂ when NO levels are high. Calculated ozone production using measured HO₂ is twice that using modeled HO₂, but this difference in calculated ozone production has minimal impact on the assessment of NO_x-sensitivity or VOC-sensitivity for midday ozone production. Evidence from this study indicates that this important discrepancy is not due to the HO₂ measurement or to the sampling of transported plumes but instead to either emissions of unknown organic species that accompany the NO emissions or unknown photochemistry involving nitrogen oxides and hydrogen oxides, possibly the hypothesized reaction OH + NO + O₂ → HO₂ + NO₂.

Vertically resolved measurements of nighttime radical reservoirs in Los Angeles and their contribution to the urban radical budget

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

Radical quantum yields from formaldehyde photolysis in the 30,400-32,890 cm(-1) (304-329 nm) spectral region: detection of radical photoproducts using pulsed laser photolysis-pulsed laser induced fluorescence

The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique across the 30,400–32,890 cm(–1) (304–329 nm) spectral region of the Ã(1)A2–X̃(1)A1 electronic transition. The photolysis laser had a bandwidth of 0.09 cm(–1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (0.07 cm(–1)), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO2. This technique produced an “action” spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm(–1). Yields were normalized to a value of 0.69 at 31,750 cm(–1) based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 5(0)(1) + 1(0)(1)4(0)(1) combination band centered at 31,125 cm(–1). This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.

Estimation and comparison of night-time OH levels in the UK urban atmosphere using two different analysis methods

Night-time OH levels have been determined for UK urban surface environments using two methods, the decay and steady state approximation methods. Measurement data from the UK National Environmental Technology Centre archive for four urban sites (Bristol, Harwell, London Eltham and Edinburgh) over the time period of 1996 to 2000 have been used in this study. Three reactive alkenes, namely isoprene, 1,3-butadiene and trans-2-pentene were chosen for the calculation of OH levels by the decay method. Hourly measurements of NO, NO2, O3, CO and 20 VOCs were used to determine night-time OH level using the steady state approximation method. Our results showed that the night-time OH levels were in the range of 1 x 10(5) - 1 x 10(6) molecules/cm3 at these four urban sites in the UK. The application of a t-test of these analyses indicated that except Bristol, there was no significant difference between the OH levels found from the decay and steady state approximation methods. Night-time levels of the OH radical appeared to peak in summer and spring time tracking the night-time O3 levels which also passed through a maximum at this time.

Deciphering the role of radical precursors during the Second Texas Air Quality Study

The Texas Environmental Research Consortium (TERC) funded significant components of the Second Texas Air Quality Study (TexAQS II), including the TexAQS II Radical and Aerosol Measurement Project (TRAMP) and instrumented flights by a Piper Aztec aircraft. These experiments called attention to the role of short-lived radical sources such as formaldehyde (HCHO) and nitrous acid (HONO) in increasing ozone productivity. TRAMP instruments recorded daytime HCHO pulses as large as 32 parts per billion (ppb) originating from upwind industrial activities in the Houston Ship Channel, where in situ surface monitors detected HCHO peaks as large as 52 ppb. Moreover, Ship Channel petrochemical flares were observed to produce plumes of apparent primary HCHO. In one such combustion plume that was depleted of ozone by large emissions of oxides of nitrogen (NOx), the Piper Aztec measured a ratio of HCHO to carbon monoxide (CO) 3 times that of mobile sources. HCHO from uncounted primary sources or ozonolysis of underestimated olefin emissions could significantly increase ozone productivity in Houston beyond previous expectations. Simulations with the CAMx model show that additional emissions of HCHO from industrial flares or mobile sources can increase peak ozone in Houston by up to 30 ppb. Other findings from TexAQS II include significant concentrations of HONO throughout the day, well in excess of current air quality model predictions, with large nocturnal vertical gradients indicating a surface or near-surface source of HONO, and large concentrations of nighttime radicals (approximately30 parts per trillion [ppt] HO2). HONO may be formed heterogeneously on urban canopy or particulate matter surfaces and may be enhanced by organic aerosol of industrial or motor vehicular origin, such as through conversion of nitric acid (HNO3). Additional HONO sources may increase daytime ozone by more than 10 ppb. Improving the representation of primary and secondary HCHO and HONO in air quality models could enhance the simulated effectiveness of control strategies.

Measurements of OH reactivity and photochemical ozone production in the urban atmosphere

Measurements of OH reactivity (i.e., OH loss rates) in the troposphere are essential for discussing tropospheric OH photochemistry. In July and August 2003, we observed the total OH reactivity in a suburban area of Tokyo. More than 90% of the data of the measured OH loss rates were higher than the calculated values with simultaneously measured concentrations of various trace species even though the rate coefficient of the OH + NO2 reaction was measured by us. We concluded that this discrepancy is due to the existence of unmeasured volatile organic compounds (VOCs). We estimated the potential of the photochemical ozone production in the case of including the unknown species as VOCs and excluding the missing sink, respectively. When the unknown species were included as VOCs, the potential increases from 32% to 88%. This result indicates the photochemical production rates of ozone in the urban air are substantially greater than expected.